Traditional hearing aids are placed in a user's ear canal. The devices function to receive and amplify acoustic signals within the ear canal to yield enhanced hearing. In some devices, “behind-the-ear” units have been utilized which comprise a microphone to transduce the acoustic input into an electrical signal, some type of signal processing circuitry to modify the signal appropriate to the individual hearing loss, an output transducer (commonly referred to in the field as a “receiver”) to transduce the processed electrical signal back into acoustic energy, and a battery to supply power to the electrical components.
Increasingly, a number of different types of fully- or semi-implantable hearing instruments have been developed. By way of example, implantable devices include instruments which employ implanted electromechanical transducers for stimulation of the ossicular chain and/or oval window, instruments which utilize implanted exciter coils to electromagnetically stimulate magnets fixed within the middle ear, and instruments which utilize an electrode array inserted into the cochlea to transmit electrical signals for sensing by the auditory nerve.
In these, as well as other implanted devices, acoustic signals are received by an implantable microphone, wherein the acoustic signal is converted to an electrical signal that is employed to generate a signal to drive an actuator that stimulates the ossicular chain and/or oval window or that is applied to selected electrodes of a cochlear electrode array. As may be appreciated, such implantable hearing instrument microphones must necessarily be positioned at a location that facilitates the receipt of acoustic signals and effective signal conversion/transmission. For such purposes, implantable microphones are most typically positioned in a surgical procedure between a patient's skull and skin, at a location rearward and upward of a patient's ear (e.g., in the mastoid region).
Given such positioning, the size and ease of installation of implantable hearing instrument microphones are primary considerations in the further development and acceptance of implantable hearing instrument systems. Further, it is important that a relatively high sensitivity and flat frequency response be provided to yield a high fidelity signal. Relatedly, the componentry cost of providing such a signal is of importance to achieving widespread use of implantable systems.